This invention relates to a fluidized bed heat exchanger and more particularly, to a heat exchanger in which heat is generated by the combustion of particulate fuel in a fluidized bed.
The use of fluidized beds has long been recognized as an attractive way of generating heat. In a normal fluidized bed arrangement, air is passed through a perforated plate or grid supporting material which usually includes a mixture of a fuel material, such as high sulfur bituminous coal, and an adsorbent material for the sulfur released as a result of the combustion of the coal. As a result of the air passing through the bed, the bed behaves like a boiling liquid which promotes the combustion of the fuel. The basic advantages of such an arrangement include a relatively high heat transfer rate, substantially uniform bed temperature, combustion at relatively low temperatures, ease of handling the coal, a reduction in corrosion and boiler fouling and a reduction in boiler size.
In the fluidized bed combustion process, the coal and adsorbent are continuously introduced into the bed by suitable feeders, injectors, or the like and the spent coal and adsorbent are discharged from the lower portion of the bed, usually through a gravity drain pipe having one end registering with a discharge opening formed through the perforated support plate and the other end communicating with a screw cooler, a conveyor belt, or the like. However, in arrangements in which the size of the coal extends over a relatively large range, the relatively larger pieces of coal tend to migrate to an area above the drain pipe and form a dense area that is difficult, if not impossible, to fluidize. As a result, the large pieces of coal do not drain, but rather cause a clogging of the drain pipe and an attendant severe curtailment in the operating efficiency of the bed.